The present application claims priority to Japanese Application No. P11-306245 filed Oct. 28, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a display apparatus which comprises organic electro-luminescence devises individually having an organic light emitting layer, and a method for fabricating such display apparatus.
2. Description of the Related Art
An organic-electro-luminescence (abbreviated as EL hereinafter) device based on electro-luminescence of organic materials has an organic layer comprising an organic hole transport layer and an organic light emitting layer laminated between a lower electrode and an upper electrode, and is attracting a good deal of attention as a light emitting device capable of high-luminance light emission by low-voltage direct current drive.
Since such an organic EL device can respond as fast as within 1 xcexcsec or below, an organic EL display apparatus using such devices can be driven by simple-matrix duty operation. A problem will, however, arise in that there is a growing need for instantaneously applying a large current to the organic EL devices to ensure sufficient luminous intensity in the future situation in which the EL display apparatus will be operated at a higher duty responding to an increasing trend in the number of pixels, which is likely to damage the devices.
On the other hand in active-matrix operation, operation current can constantly be applied to the organic EL devices during a period of one frame according to signal voltage, since the signal voltage can be retained by a retention capacitor provided to the individual pixels together with a thin film transistor (abbreviated as TFT hereinafter). Thus, unlikely to a case with the single matrix operation, there is no need to instantaneously apply a large current so that the damages to the organic EL devices can be reduced.
In the active-matrix display apparatus using such organic EL devices (that is, the organic EL display), each pixel on a substrate is individually provided with a TFT, the TFTs are covered with an interlayer insulating film, and further thereon organic EL devices are formed. Each organic EL device comprises a lower electrode patterned for the individual pixels so as to be connected to the TFT, an organic layer formed so as to cover the lower electrode, and an upper electrode provided so as to cover the organic layer.
In such an active-matrix display apparatus, the upper electrode is formed as a so-called blanket film covering all pixels, and used as an upper common electrode for such all pixels. In the display apparatus capable of color display, the organic layers are separately patterned for each color on the lower electrode.
Such display apparatus having organic EL devices on the insulating film covering the TFTs on the substrate is, however, disadvantageous in that such TFTs are causative of narrowing the aperture of the organic EL devices when the display apparatus is designed as of transmission type in which light emitted from the organic layer is observed from the substrate side.
Thus for the active-matrix display apparatus, it is supposed to be advantageous to adopt a so-called upper light withdrawing structure (hereinafter referred to as top emission type) in which the light is withdrawn from an opposite side of the substrate so as to ensure a sufficient aperture of the organic EL devices.
Constituting the active-matrix display apparatus as the top emission type requires the lower electrode to be made of a light reflective material and the upper common electrode to be made of a transparent material. Indium tin oxide (ITO) and indium zinc oxide (IXO), known as materials for transparent conductive film, are however higher in resistivity than metals, so that the upper common electrode is likely to cause voltage drop due to internal voltage gradient occurs therein. This may cause significant degradation of the display properties since the voltage applied to the individual organic EL devices on the display plane becomes non-uniform, and since the luminous intensity in the center portion of the display plane decreases.
While the transparent conductive film such as ITO or IXO can be formed by evaporation or sputtering, the former method is difficult to yield a film of good quality, and the obtained film tends to have high resistivity and low light transmissivity. The sputtering is thus adopted in the fabrication process of the display apparatus to form the transparent conductive film. The sputtering is, however, higher in energy of particles to be deposited as compared with vapor deposition, so that the underlying surface on which the deposition proceeds is more likely to be damaged. Since the basic structure of the organic EL device resembles to that of a light emitting diode made of inorganic semiconductor materials, such damages exerted on the underlying organic layer will be causative of leak current, which may further result in the production of non-emissive pixels called xe2x80x9cdead pixelsxe2x80x9d.
To avoid the production of such dead pixels, it is proposed to form the upper common electrode with a metal film thin enough so as to allow sufficient light transmissivity. Such metal film may, however, inevitably has a high sheet resistance as a result of the thinning and will also produce voltage gradient in the upper common electrode similarly to the case with the transparent conductive film, which will cause voltage drop and thus will significantly lower the display properties.
The thinning of the upper common electrode raises another problem in that the electrode cannot fully prevent the atmospheric moisture or oxygen from intruding into the organic layer, which will accelerate deterioration of the organic layer.
It is therefore an object of the present invention to provide an active-matrix-type display apparatus ensuring sufficient luminous intensity of the organic EL devices and allowing improved display properties.
To accomplish the foregoing object, a first aspect of the present invention relates to a display apparatus having a plurality of pixels, which comprises:
a first electrode formed on a substrate;
a light emitting layer formed on the first electrode; and
a second electrode formed on the light emitting layer, wherein:
the plurality of pixels are partitioned by a rib larger in thickness than the light emitting layer and having at least a conductive material; and the conductive material layer is electrically connected to the second electrode.
Since the rib having the conductive material serves as an auxiliary wiring for the second electrode, the pressure drop of the second electrode can successfully be suppressed and thus sufficient luminous intensity of the organic light emitting layer of the individual pixels can be retained even when the second electrode is made of a high-resistivity material. Moreover, the rib also serves as a spacer for a mask used for patterning the organic layer, so that there is no need to separately provide the spacer and the auxiliary wiring between every adjacent pixels. This allows space saving between every adjacent pixels and reserve of the pixel area.
According to a second aspect of the present invention, there is provided a display apparatus having a plurality of pixels, which comprises:
a field effective transistor formed on a substrate and having a first electrode, a second electrode and a third electrode;
an interlayer insulating film formed on the field effective transistor;
a lower electrode connected to the first electrode through an opening formed on the interlayer insulating film;
an organic layer formed on the lower electrode and having a light emitting layer; and
an upper electrode formed on the organic layer, wherein:
the plurality of pixels are partitioned by a rib larger in thickness than the organic layer and having at least a conductive material; and the conductive material layer is electrically connected to the upper electrode.
According to the display apparatus according to the second aspect of the present invention, sufficient luminous intensity of the organic light emitting layer of all pixels on the display plane can be retained while achieving the space saving between every adjacent pixels to reserve sufficient pixel area by providing between every adjacent pixels the rib which serves as the spacers for the mask used for patterning the organic layer and also as the auxiliary electrode for the upper electrode covering the entire plane of the display plane. This results in improvement in the display properties of an active-matrix-type display apparatus.
According to a third aspect of the present invention, there is provided a method for fabricating a display apparatus having a plurality of pixels, which comprises:
a step for forming on a substrate a first electrode;
a step for forming between adjacent ones of the plurality of pixels a rib having a conductive material;
a step for placing a mask on the rib, and forming on the first electrode a light emitting layer smaller in thickness than the rib; and
a step for forming a second electrode on the light emitting layer and on the rib.
According to a fourth aspect of the present invention, there is provided a method for fabricating a display apparatus having a plurality of pixels, which comprises:
a step for forming on a substrate a field effective transistor having a first electrode, a second electrode and a third electrode;
a step for forming on the field effective, transistor an interlayer insulating film;
a step for forming an opening to the interlayer insulating film;
a step for forming on the interlayer insulating film a lower electrode connected through the opening to the first electrode;
a step for forming between adjacent ones of a plurality of the pixels a rib having a conductive material;
a step for placing a mask on the rib, and forming on the lower electrode an organic layer having a light emitting layer smaller in thickness than the rib; and
a step for forming on the organic layer an upper electrode and a protective layer made of an insulating material or a conductive material while keeping the organic layer unexposed to the air.